Editing Rare-earth element (section)

===Spectroscopic identification===
There were no further discoveries for 30 years, and the element [[didymium]] was listed in the periodic table of elements with a molecular mass of 138. In 1879, [[Marc Delafontaine|Delafontaine]] used the new physical process of [[atomic emission spectroscopy|optical flame spectroscopy]] and found several new spectral lines in didymia. Also in 1879, [[Paul Émile Lecoq de Boisbaudran]] isolated the new element ''[[samarium]]'' from the mineral [[samarskite]].

In 1886, the samaria earth was further separated by Lecoq de Boisbaudran. A similar result was obtained by [[Jean Charles Galissard de Marignac]] by direct isolation from samarskite. They named the element ''[[gadolinium]]'' after [[Johan Gadolin]], and its oxide was named "[[gadolinium(III) oxide|gadolinia]]".

Further spectroscopic analysis between 1886 and 1901 of samaria, yttria, and samarskite by [[William Crookes]], Lecoq de Boisbaudran and [[Eugène-Anatole Demarçay]] yielded several new [[spectral line]]s that indicated the existence of an unknown element. In 1901, the [[fractional crystallization (chemistry)|fractional crystallization]] of the oxides yielded ''[[europium]]''.

In 1839, the third source for rare earths became available. This is a mineral similar to gadolinite called ''uranotantalum'', now called "[[samarskite]]", an oxide of a mixture of elements such as yttrium, ytterbium, iron, uranium, thorium, calcium, niobium, and tantalum. This mineral from [[Miass]] in the southern [[Ural Mountains]] was documented by [[Gustav Rose]]. The Russian chemist R. Harmann proposed that a new element he called "[[ilmenium]]" should be present in this mineral, but later, [[Christian Wilhelm Blomstrand]], Galissard de Marignac, and [[Heinrich Rose]] found only [[tantalum]] and [[niobium]] ([[columbium]]) in it.

The exact number of rare-earth elements that existed was highly unclear, and a maximum number of 25 was estimated. Using [[X-ray emission spectroscopy|X-ray spectra]] [[Henry Moseley|Henry Gwyn Jeffreys Moseley]] confirmed the atomic theory of [[Niels Bohr]] and simultaneously developed the theory of atomic numbers for the elements.<ref>{{Cite book |last=Heilbron |first=J. L. |title=H. G. J. Moseley: the life and letters of an English physicist, 1887-1915 |last2=Moseley |first2=H. G. J. |date=1974 |publisher=University of California Press |isbn=978-0-520-02375-8 |location=Berkeley}}</ref> Moseley found that the exact number of lanthanides had to be 15, revealing a missing element, [[promethium|element 61]], a radioactive element with a half-life of 18 years.<ref>{{Cite web |title=Separation of Rare Earth Elements |url=https://www.acs.org/education/whatischemistry/landmarks/earthelements.html |access-date=2025-04-02 |website=American Chemical Society |language=en}}</ref>

Using these facts about atomic numbers from X-ray crystallography, Moseley also showed that [[hafnium]] (element 72) would not be a rare-earth element. Moseley was killed in [[World War I]] in 1915, years before hafnium was discovered. Hence, the claim of [[Georges Urbain]] that he had discovered element 72 was untrue. Hafnium is an element that lies in the periodic table immediately below [[zirconium]], and hafnium and zirconium have very similar chemical and physical properties.